1. Field of the Invention
The present invention relates to a micrometer. More specifically, the present invention relates to a micrometer of which frame deformation in accordance with temperature change in using environment can be restrained as small as possible.
2. Description of Related Art
Conventionally, as shown in FIG. 3, an ordinary micrometer has a U-shaped frame 1, an anvil installed on one end of the frame 1, a feeding mechanism 3 (ordinarily, such as a feed screw) installed on the other end of the frame 1 and a spindle 4 to move forward and backward to the anvil 2.
The micrometer is ideally fixed to a stand or the like under an environment of a standard temperature (20.degree. C., for example) in measuring a workpiece. However, the micrometer is often used under environments outside the standard temperature being held by hand. In other words, while holding the frame by one hand, the spindle 4 is advanced to the anvil 2 by handling the feeding mechanism 3 to hold the workpiece W between the spindle 4 and the anvil 2. At this state, the moving displacement amount of the spindle 4 is read from the scale (or graduation) 5.
When the micrometer is held by hand under an environment outside the standard temperature, the frame 1 is thermally deformed and the spindle is stretched or contracted by the circumambient temperature and heat of the hand, thereby deteriorating measurement accuracy.
As an instant case, the micrometer is held by one hand under an environment lower than the standard temperature. When the micrometer is thermally stabilized in advance to adjust the mechanical origin under an environment of standard temperature, the frame 1 has a configuration shown in double-dotted line in FIG. 4.
When a U-shaped bent portion 6 is held by hand to measure the workpiece, a temperature distribution of the frame 1 is changed to deform the frame 1 as shown in a solid line in FIG. 4, causing a frame deformation error of .DELTA.x. Furthermore, the spindle 4 is contracted to cause additional error to magnify an entire error.
To solve above disadvantage, a material having an extremely small linear thermal expansion coefficient may be employed to both of the frame 1 and the spindle 4. However, it is not practical in view of production cost or the like.